| 1. |
- Agrell, Johan, 1974-
(författare)
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Development of Methanol-Reforming Catalysts for Fuel Cell Vehicles
- 2003
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Vehicles powered by proton exchange membrane (PEM) fuelcells are approaching commercialisation. Being inherently cleanand efficient sources of power, fuel cells constitute asustainable alternative to internal combustion engines to meetfuture low-emission legislation. The PEM fuel cell may befuelled directly by hydrogen, but other alternatives appearmore attractive at present, due to problems related to theproduction, transportation and handling of hydrogen.Fuelling with an alcohol fuel, such as methanol, which isoxidised directly at the anode, offers certain advantages.However, the efficiency of the direct-methanol fuel cell (DMFC)is still significantly lower than that of the conventionalhydrogen-fuelled PEM fuel cell, due to some technical problemsremaining unsolved. Hence, indirect fuelling by a reformedliquid fuel may be the most feasible option in the early stagesof the introduction of fuel cell vehicles.The work presented in this thesis concerns the developmentof catalysts for production of hydrogen from methanol bypartial oxidation, steam reforming or a combination thereof.The work contributes to the understanding of how thepreparation route affects catalyst morphology and howphysicochemical properties determine catalytic behaviour andreaction pathways.The thesis is a summary of seven papers published inscientific periodicals. The first paper (Paper I) reviews thecurrent status of catalytic hydrogen generation from methanol,focusing on the fuel cell application. Paper II investigatesthe partial oxidation of methanol over Cu/ZnO catalystsprepared in microemulsion and by a conventionalco-precipitation technique. The activity for methanolconversion in the low-temperature regime is found to besignificantly higher over the former materials and the workcontinues by determining the nature of possible Cu-ZnOinteractions in the catalysts by studying their physicochemicalproperties more thoroughly (Paper III). In Paper IV, thepathways for methanol conversion via both partial oxidation andsteam reforming are elucidated.In Paper V, partial oxidation of methanol is studied overPd/ZnO catalysts prepared by microemulsion technique and againcompared to conventional materials. This investigationdemonstrates that although possessing high methanol conversionactivity, palladium-based catalysts are not suitable forreforming in fuel cell applications due to the considerableamounts of carbon monoxide formed.In Paper VI, methanol reforming is investigated over acommercial Cu/ZnO/Al2O3 catalyst. The mechanisms for carbonmonoxide formation and strategies for its suppression arediscussed, as well as reactor design aspects. The study alsoincludes some simple kinetic modelling. Finally, Paper VIIdescribes the optimisation of catalyst composition and processconditions to reach high hydrogen production efficiency at lowoperating temperatures and with minimum carbon monoxideformation.Keywords:PEM fuel cells, hydrogen, methanol, reforming,(partial) oxidation, reaction pathways, carbon monoxide,catalyst, microemulsion, Cu/ZnO, Pd/ZnO, copper, redoxproperties, oxidation state
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| 2. |
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| 3. |
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| 4. |
- Andrae, Johan
(författare)
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Numerical Studies of Wall Effects of Laminar Flames
- 2001
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Numerical simulations have been done with the CHEMKINsoftware to study different aspects of wall effects in thecombustion of lean, laminar and premixed flames in anaxisymmetric boundary-layer flow. The importance of the chemical wall effects compared to thethermal wall effects caused by the development of the thermaland velocity boundary layer has been investigated in thereaction zone by using different wall boundary conditions, walltemperatures and fuel/air ratios. Surface mechanisms include acatalytic surface (Platinum), a surface that promotesrecombination of active intermediates and a completely inertwall with no species and reactions as the simplest possibleboundary condition. When hydrogen is the model fuel, the analysis of the resultsshow that for atmospheric pressure and a wall temperature of600 K, the surface chemistry gives significant wall effects atthe richer combustion case (f=0.5), while the thermal andvelocity boundary layer gives rather small effects. For theleaner combustion case (f=0.1) the thermal and velocityboundary layer gives more significant wall effects, whilesurface chemistry gives less significant wall effects comparedto the other case. For methane as model fuel, the thermal and velocity boundarylayer gives significant wall effects at the lower walltemperature (600 K), while surface chemistry gives rather smalleffects. The wall can then be modelled as chemically inert forthe lean mixtures used (f=0.2 and 0.4). For the higher walltemperature (1200 K) the surface chemistry gives significantwall effects. For both model fuels, the catalytic wall unexpectedlyretards homogeneous combustion of the fuel more than the wallthat acts like a sink for active intermediates. This is due toproduct inhibition by catalytic combustion. For hydrogen thisoccurs at atmospheric pressure, but for methane only at thehigher wall temperature (1200 K) and the higher pressure (10atm). As expected, the overall wall effects (i.e. a lowerconversion) were more pronounced for the leaner fuel-air ratiosand at the lower wall temperatures. To estimate a possible discrepancy in flame position as aresult of neglecting the axial diffusion in the boundary layerassumption, calculations have been performed with PREMIX, alsoa part of the CHEMKIN software. With PREMIX, where axialdiffusion is considered, steady, laminar, one-dimensionalpremixed flames can be modelled. Results obtained with the sameinitial conditions as in the boundary layer calculations showthat for the richer mixtures at atmospheric pressure the axialdiffusion generally has a strong impact on the flame position,but in the other cases the axial diffusion may beneglected. Keywords:wall effects, laminar premixed flames,platinum surfaces, boundary layer flow
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| 5. |
- Andrae, Johan, 1973-
(författare)
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Wall Related Lean Premixed Combustion Modeled with Complex Chemistry
- 2002
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Increased knowledge into the physics and chemistrycontrolling emissions from flame-surface interactions shouldhelp in the design of combustion engines featuring improvedfuel economy and reduced emissions. The overall aim of this work has been to obtain afundamental understanding of wall-related, premixed combustionusing numerical modeling with detailed chemical kinetics. Thiswork has utilized CHEMKIN®, one of the leading softwarepackages for modeling combustion kinetics. The simple fuels hydrogen and methane as well as the morecomplex fuels propane and gasified biomass have been used inthe model. The main emphasis has been on lean combustion, andthe principal flow field studied is a laminar boundary layerflow in two-dimensional channels. The assumption has been madethat the wall effects may at least in principle be the same forlaminar and turbulent flames. Different flame geometries have been investigated, includingfor example autoignition flames (Papers I and II) and premixedflame fronts propagating toward a wall (Papers III and IV).Analysis of the results has shown that the wall effects arisingdue to the surface chemistry are strongly affected by changesin flame geometry. When a wall material promoting catalyticcombustion (Pt) is used, the homogeneous reactions in theboundary layer are inhibited (Papers I, II and IV). This isexplained by a process whereby water produced by catalyticcombustion increases the rate of the third-body recombinationreaction: H+O2+M ⇔ HO2+M. In addition, the water produced at higherpressures increases the rate of the 2CH3(+M) ⇔ C2H6(+M) reaction, giving rise to increased unburnedhydrocarbon emissions (Paper IV). The thermal coupling between the flame and the wall (theheat transfer and development of the boundary layers) issignificant in lean combustion. This leads to a sloweroxidation rate of the fuel than of the intermediatehydrocarbons (Paper III). Finally in Paper V, a well-known problem in the combustionof gasified biomass has been addressed, being the formation offuel-NOx due to the presence of NH3 in the biogas. A hybridcatalytic gas-turbine combustor has been designed, which cansignificantly reduce fuel-NOx formation. Keywords:wall effects, premixed flames, flamequenching, numerical modeling, CHEMKIN, boundarylayerapproximation, gasified biomass, fuel-NOx, hybrid catalytic combustor.
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| 6. |
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| 7. |
- Anheden, Marie
(författare)
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Analysis of gas turbine systems for sustainable energy conversion
- 2000
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Increased energy demands and fear of global warming due tothe emission of greenhouse gases call for development of newefficient power generation systems with low or no carbondioxide(CO2) emissions. In this thesis, two different gasturbine power generation systems, which are designed with theseissues in mind, are theoretically investigated and analyzed.Inthe first gas turbine system, the fuel is combusted using ametal oxide as an oxidant instead of oxygen in the air. Thisprocess is known as Chemical Looping Combustion (CLC). CLC isclaimed to decrease combustion exergy destruction and increasethe power generation efficiency. Another advantage is thepossibility to separate CO2without a costly and energy demanding gasseparation process. The system analysis presented includescomputer-based simulations of CLC gas turbine systems withdifferent metal oxides as oxygen carriers and different fuels.An exergy analysis comparing the exergy destruction of the gasturbine system with CLC and conventional combustion is alsopresented. The results show that it is theoretically possibleto increase the power generation efficiency of a simple gasturbine system by introducing CLC. A combined gas/steam turbinecycle system with CLC is, however, estimated to reach a similarefficiency as the conventional combined cycle system. If thebenefit of easy and energy-efficient CO2separation is accounted for, a CLC combined cyclesystem has a potential to be favorable compared to a combinedcycle system with CO2separation.In the second investigation, a solid, CO2-neutral biomass fuel is used in a small-scaleexternally fired gas turbine system for cogeneration of powerand district heating. Both open and closed gas turbines withdifferent working fluids are simulated and analyzed regardingthermodynamic performance, equipment size, and economics. Theresults show that it is possible to reach high power generationefficiency and total (power-and-heat) efficiency with thesuggested system. The economic analysis reveals that the costof electricity from theEFGT plant is competitive with the moreconventional alternatives for biomass based cogeneration in thesame size range (<10 MWe).Keywords:power generation, Chemical Looping Combustion,CO2separation, oxygen carrier, biomass fuel, closedcycle gas turbine, externally fired gas turbine
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| 8. |
- Arnerdal, Klas
(författare)
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Transport of Heavy Metals in Soil Subjected to an Electric Field -A Study of the Transport of Copper in Bentonite Clay
- 2002
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This licentiate thesis addressesphosphorus release from sewage sludge, which is the first stepin phosphorus recovery. The concept ofphosphorus recovery maybe seen as a part of sustainable sludge handling in that it isa means of recycling resources without supplying harmfulsubstances to humans or the environment. Independent of whichdirection the future development in the water and wastewatersector turns, phosphorus recovery as a part of the wholewastewater system is presumably to be included.In order to obtain moreinformation about different ways of phosphorus release fromsewage sludge experiments have been carried out. The results ofthe experiments on phosphorus release using acid and base arepresented. Fractionation of sludge is an important step torelease different components from the sludge, includingphosphorus. With the addition of acid, a large fraction of theinorganic material can be dissolved, and thereby also sludgebound phosphorus compounds. In particular, fractionation byusing acid treatment on digested sludge releases a considerableamount of phosphorus.The method of phosphorus removalin a conventional wastewater treatment plant has an impact onthe chemicals required for phosphorus recovery. A comparison oftechnology for the phosphorus recovery systems is highlydependent on the composition of the sludge, which is the rawmaterial for recovery of phosphorus, and different localconditions.The results of the performedexperiments concerning chemical demand for phosphorus releaseare compared with calculations based on a study of chemicaldemand of different technology systems for phosphorus recoveryat present operational conditions at Henriksdal wastewatertreatment plant. Overall, experiment and theory seem to be ingood agreement. Thermal technology and addition of acid torelease phosphorus lead to high chemical demand. The essentialfactor is the incoming iron content of the sludge, as given asthe quotient iron/phosphorus.Another possible method torelease phosphorus may be to use supercritical water oxidation,followed by phosphorus recovery. Studies of the Aqua Reciprocess, which uses this principle, have recently begun and theprocess is under development. Experiments have been performedto give information about recovery of phosphorus from sewagesludge, with application to digested sludge from biologicaltreatment and chemical precipitation with iron. Experimentsperformed on sludge residue from supercritical water oxidationare presented in this thesis.The results show that phosphatewas leached out more easily with acid (hydrochloric acid) atroom temperature than with base (sodium hydroxide), howeverheavy metals show slightly higher leaching percentage with acidthan with base at room temperature. Also, the leachingexperiment showed that the phosphate is leached out before theiron, in both cases. The iron content seems to be leached outto a somewhat greater degree with acid than with base. This maydisturb the further process to obtain a valuable purephosphorus product. The results also indicate that phosphorusis more easily leached out with acid from the sludge residuefrom supercritical water oxidation than from ash viaco-incineration with sewage sludge and household wastes.Supercritical water oxidation followed by phosphorus recoverymay be seen as an option to incineration of sludge andphosphorus recovery from ash.Keywords:Chemical demand, phosphorus recovery, sludgefractionation, supercritical water oxidation, sustainablesludge handling
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| 9. |
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| 10. |
- Bartlett, Michael, 1975-
(författare)
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Developing Humidified Gas Turbine Cycles
- 2002
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- As a result of their unique heat recovery properties,Humidified Gas Turbine (HGT) cycles have the potential todeliver resource-effective energy to society. The EvaporativeGas Turbine (EvGT) Consortium in Sweden has been studying thesetypes of cycles for nearly a decade, but now stands at acrossroads, with commercial demonstration remaining. Thisthesis binds together several key elements for the developmentof humidified gas turbines: water recovery and air and waterquality in the cycle, cycle selection for near-term, mid-sizedpower generation, and identifying a feasible niche market fordemonstration and market penetration. Moreover, possiblesocio-technical hinders for humidified gas turbine developmentare examined.Through modelling saltcontaminant flows in the cycle andverifying the results in the pilot plant, it was found thathumidification tower operation need not endanger the hot gaspath. Moreover, sufficient condensate can be condensed to meetfeed water demands. Air filters were found to be essential tolower the base level of contaminant in the cycle. This protectsboth the air and water stream components. By capturing airparticles of a similar size to the air filters, the humidifieractually lowers air stream salt levels. Measures to minimisedroplet entrainment were successful (50 mg droplets/kg air) andmodels predict a 1% blow down from the water circuit issufficient. The condensate is very clean, with less than 1 mg/lalkali salts and easily deionised.Based on a core engine parameter analysis for three HGTcycle configurations and a subsequent economic study, asteam-cooled steam injected cycle complemented with part-flowhumidification is recommended for the mid-size power market.This cycle was found to be particularly efficient at highpressures and turbine inlet temperatures, conditions eased bysteam cooling and even intercooling. The recommended HGT cyclegives specific investment costs 30- 35% lower than the combinedcycles and cost of electricity levels were 10-18% lower.Full-flow intercooled EvGT cycles give high performances, butseem to be penalised by the recuperator costs, while stillbeing cheaper than the CC. District heating is suggested as asuitable niche market to commercially demonstrate the HGTcycle. Here, the advantages of HGT are especially pronounceddue their very high total efficiencies. Feasibility prices forelectricity were up to 35% lower than competing combinedcycles. HGT cycles were also found to effectively include wasteheat sources.Keywords:gas turbines, evaporative gas turbines,humidification, power generation, combined heat and powergeneration.
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| 11. |
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| 12. |
- Berg, Magnus, 1965-
(författare)
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Catalytic Oxidation of Emissions from Residential Wood fired boilers
- 2001
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The European Commission has set a target of doubling theenergy supply from renewable sources from 6% in 1997 to 12% in2010. An increased usage of biofuels will play an importantrole in meeting this target. The Swedish government is alsoactively supporting a changeover from nuclear power to anincreased use of biofuels but have at the same time set "cleanair" environmental goals that must not be obstructed by theincreased usage of biofuels. To be able to simultaneously meetthese objectives, the development of new and improvedtechnologies for low emission equipment must be continued.Biofuels are today utilised in large-scale combined heat andpower plants as well as in residential heating, especially thelatter is often connected with high levels of harmfulemissions. This thesis describes the utilisation of oxidationcatalysts for the abatement of emissions from residentialboilers, which is one promising technique for the developmentof low emission boilers. The overall aim has been to developthis technique to the stage of commercial manufacture ofcatalytic boilers.The first part of the thesis reviews the use of biofuels inresidential heating with special emphasis on the emissions fromsuch appliances. Some general aspects of applied catalysis arediscussed and previous experience from the use of oxidationcatalysts in wood fired stoves are summarised.The second part of the thesis consists of a summary anddiscussion of the experimental work on oxidation catalysts inwood fired boilers, papersI-VI. Three different boilers were used for theseinvestigations, a traditional over-fired boiler based onnatural draught, a down-draught boiler using forced airsupplyand a modern down-draught boiler equipped with a flue gas fan.In paperIpellet based catalysts were evaluated, papersII-Vfocused on monolithic catalysts and in paperVIthe use of net-based catalysts were evaluated.The conclusion is that the integration of monolithiccatalysts in modern down-draught boilers has a large potentialfor the development of low emission wood fired boilers. It isshown that the emissions of carbon monoxide and hydrocarbons,with the exception of methane, is oxidised effectively over thecatalyst. A number of important aspects for optimal integrationof the catalyst in the boiler are demonstrated, as well as theeffect of combining primary measures with oxidation catalysts.The technology is at a stage where field trials are the nextstep for answering the main remaining issue, the lifetime ofthe catalyst.Keywords:biomass, wood, combustion, residentialboilers, applied catalysis, catalytic oxidation, VOC, carbonmonoxide
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| 13. |
- Birgersson, Henrik, 1976-
(författare)
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Development and Assessment of Regeneration Methods for Commercial Automotive Three-Way Catalysts
- 2006
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Car exhaust catalysts were introduced in the early 1980’s, to limit the release of pollutants such as hydrocarbons, carbon monoxide and nitrogen oxides. These catalysts contain noble metals such as palladium (Pd), platinum (Pt) and rhodium (Rh) and are able to simultaneously abate all three of the above-mentioned pollutants, hence the name three-way catalyst (TWC). The exposure to high temperatures (800-1000 °C) during operation and the presence of additives in gasoline and lubricants will, after a certain time, lower the activity of the TWC. High temperatures reduce the active area by causing the noble metals to agglomerate and sinter, whereas the additives alter the activity either by fouling the pores of the support material or by interacting with the metals. The main objective of this work was to develop a method which allows for the removal of contaminants (additives) from the washcoat and enables the redispersion of the active sites (noble metals), in an effort to recover lost catalyst activity. For this purpose, regeneration experiments were carried out on a wide spectrum of different commercial car exhaust catalysts. The influence of a thermal treatment in a controlled gas atmosphere, such as oxygen or hydrogen, and a redispersing agent, e.g. chlorine, on the activity of TWC was investigated by means of laboratory-scale activity measurements. Several complementary characterization methods such as SEM/TEM, XRD, BET, LA, XPS and TPR were employed to verify the effects of the regeneration treatments on the catalyst morphology (Papers I, II). The results show that partial regeneration of catalyst activity and noble metal dispersion was achieved after thermal treatment in an oxygen-chlorine rich atmosphere at temperatures below 500 °C. A wet-chemical regeneration treatment with dilute oxalic and citric acid solutions is evaluated in Paper III. These acidic solutions are able to dissolve and remove contaminants from the washcoat, thus partly restoring the catalyst activity. An investigation of the effects of an oxy-chlorine thermal treatment for regeneration of a ‘full-scale’ commercial automotive three-way catalyst was carried out (Paper IV). Improved catalyst activity for a high mileage catalyst could be observed, with emissions lowered by approximately 30 to 40 vol.% over the EC2000 driving cycle. The properties of fresh, aged and regenerated catalysts were then studied by means of labscale experiments, on a local as well as a global level using a mathematical model (Paper V). The model allows for comparison of the intrinsic properties of the active surface by deriving and tuning parameters of a fresh catalyst and verifying the activity of a regenerated or aged catalyst.
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| 14. |
- Birgersson, Henrik
(författare)
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Development of a regeneration procedure for commercial automotive three-wy catalysts
- 2004
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Car exhaust catalysts were introduced in the early 1980’s, to limit the release of pollutants such as hydrocarbons, carbon monoxides and nitrogen oxides. These catalysts contain noble metals such as palladium (Pd), platinum (Pt) and rhodium (Rh) and are able to simultaneously abate all three of the above-mentioned pollutants, hence the name three-way catalyst (TWC). The exposure to high temperatures (800-1000 °C) during operation and the presence of additives in petrol such as lead, calcium, silicon, magnesium, manganese, chromium, sulphur and phosphorus will after a certain time start to lower the overall effectiveness of the catalyst. These effects are either of a mechanical or a chemical nature. High temperatures reduce the active area by causing the noble metals to agglomerate and sinter whereas the additives alter the activity by either fouling the pores of the support material (phosphorus) or by interacting with the metals (sulphur and lead). The main objective of this work was to develop a method to redisperse the catalytically active sites, comprising Pd, Pt and Rh on the washcoat surface, in an effort to regain lost catalyst activity. For this purpose, a wide spectrum of different commercial car exhaust catalysts containing varying noble metal loadings, aged under various driving conditions and with mileages ranging from 30 to 100 000 km were evaluated. The influence of a thermal treatment in a controlled gas atmosphere, such as oxygen or hydrogen and a redispersing agent, e.g. chlorine, on the activity of TWC was investigated by means of laboratory-scale activity measurements. Several complementary characterisation methods such as SEM/TEM, XRD, BET and TPR were used to verify the effects of the regeneration treatments on the catalyst morphology (Paper I). Partial regeneration of catalyst activity and noble metal dispersion was achieved after thermal treatment in an oxygen-chlorine rich atmosphere at temperatures below 500 °C. Finally, an investigation of the effects of an oxy-chlorine thermal treatment for regeneration of a ‘full-scale’ commercial automotive three-way catalyst was performed. Catalyst activity and performance prior to and after the oxy-chlorine thermal treatment was measured on a test vehicle in accordance with the European driving cycle (EC2000). The catalyst surface was further characterised using XRD and EDX (Paper II). Improved catalyst activity for a high mileage catalyst could be observed, with emissions lowered by approximately 30 to 40 vol% over the EC2000 driving cycle
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| 15. |
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| 16. |
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| 18. |
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| 19. |
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| 20. |
- Byrne, Philip, 1971-
(författare)
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Mathematical modelling and experimental simulation of chlorate and chlor-alkali cells.
- 2001
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The production of chlorate, chlorine and sodium hydroxiderelies heavily on electrical energy, so that savings in thisarea are always a pertinent issue. This can be brought aboutthrough increased mass transfer of reacting species to therespective electrodes, and through increased catalytic activityand uniformity of current density distribution at theseelectrodes. This thesis will present studies involvingmathematical modelling and experimental investigations of theseprocesses. They will show the effect that hydrodynamicbehaviour has on the total current density and cell voltages,along with the effects on current density distributions andindividual overpotentials atthe respective electrodes.Primary, secondary and psuedo-tertiary current densitydistribution models of a chlor-alkali anode are presented anddiscussed. It is shown that the secondary model presentsresults rather similar to the pseudo-tertiary model, when thecurrent density distribution is investigated, although thepotential distribution differs rather markedly. Furthermore, itis seen that an adequate description of the hydrodynamicsaround the anode is required if the potential distribution, andthereby the prevalence of side-reactions, is to be reasonablepredicted.A rigorous tertiary current density distribution model ofthe chlorate cell is also presented, which takes into accountthe developing hydrodynamic behaviour along the height of thecell. This shows that an increased flowrate gives more uniformcurrent density distributions. This is due to the fact that theincreased vertical flowrate of electrolyte replenishes ioncontent at the electrode surfaces, thus reducing concentrationoverpotentials. Furthermore, results from the model lead to theconclusion that it is the hypochlorite ion that partakes in themajor oxygen producing side-reaction.A real-scale cross-section of a segmented anode-cathode pairfrom a chlorate cell was designed and built in order to studythe current density distribution in industrial conditions.These experiments showed that increased flowrate brought aboutmore even current density distributions, reduced cell voltageand increased the total current density. An investigation ofthe hydrodynamic effects on the respective electrodeoverpotentials shows the anode reactions being more favoured byincreased flowrate. This leads to the conclusion that theuniform current density distribution, caused by increasedflowrate, occurs primarily through decreasing the concentrationoverpotential at the anode rather than by decreasing thebubble-induced ohmic drop at the cathode.Finally, results from experiments investigating thebubble-induced free convection from a small electrochemicalcell are presented. These experiments show that Laser DopplerVelocimetry is the most effective instrument for investigatingthe velocity profiles in bubble-containing electrochemicalsystems. The results also show that the flow can transform fromlaminar to turbulent behaviour on both the vertical andhorizontal planes, in electrochemical systems where bubbles areevolved.
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| 21. |
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| 23. |
- Crawford, James
(författare)
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A model of pH and redox buffer depletion in waste landfills
- 1999
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The geochemical processes governing the release of heavymetal contaminants from solid waste materials have been studiedexperimentally and modelled using a mass balance approach. Theaim of the modelling work was to develop calculation tools thatcan be used to predict the release rate of heavy metals fromwaste heaps or landfills. The leaching models are based uponthe observation that the pH and redox state of the water incontact with waste are the two master variables that controlheavy metal mobility in aqueous environments. The modelsdeveloped are intended to be simple and sufficiently flexiblethat they may be adapted to different waste types withreasonable ease.The depletion rates of pH and redox buffering constituentsin the waste material are calculated by coupling a descriptionof the geochemical processes occurring within the waste withthe in- and out-transport of reactive chemical species. Themodels of waste leaching require input data to describe thereactive properties of the waste. These data may be obtainedfrom simple laboratory experiments such as pH static titrationsand reducing capacity measurements.The simulation results indicate that oxidation of organicmaterial can improve the quality of solid wastes by way ofcarbonation processes that transform highly soluble oxide,hydroxide, and silicate forms of Ca into calcite. Calcitebuffers leachate at a circumneutral pH level that is optimalfor the minimisation of heavy metal leaching. Oxidation oforganic material can occur under oxic conditions, or underanoxic conditions using sulphate as an oxidant. Under theanoxic conditions that prevail in a fully water saturatedleaching environment, reduction of sulphate also promotes theformation of insoluble heavy metal sulphide minerals. It isconcluded that disposal of waste materials in a fully watersaturated environment is the best way of minimisingenvironmental impact from these potentially hazardousmaterials.Keywords: Leaching, chemical weathering, acidneutralisation, solid waste, combustion residues, pH buffering,redox buffering, geochemical modelling, reactive transport
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| 24. |
- Dalili, Farnosh, 1969-
(författare)
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Humidification in Evaporative Power Cycles
- 2003
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Evaporative gas turbine cycles (EvGT) show an exceptionalexhaust heat recovery potential, which makes them a strongcompetitor to other advanced gas turbine cycles, especiallyfrom small to intermediate sizes. Evaporative gas turbines aredistinguished by humidifying the working fluid beforecombustion at temperatures below the boiling point of water;and the heat required for evaporation of water is partly takenout of the exhaust gas. Thus, humidification is a key operationin these cycles. This thesis investigates, both theoreticallyand experimentally, two alternative approaches tohumidification: the packed-bed humidification tower and thetubular humidifier. Both these equipments involvecountercurrent contact between water and the working fluid.Humidifier design criteria are developed and criticalparameters such as flooding, wetting rate and entrainment arediscussed. The experimental parts were carried out on thepacked-bed tower in the EvGT pilot plant, and on a tubularhumidifier test rig especially erected for this purpose. Thetheoretical models were confirmed by the experiments.The height of a transfer unit, necessary for designingpacked beds, was calculated for the packing employed in theEvGT pilot plant. It was found that the data provided by themanufacturer may be used with minor corrections.The tubular test rig operated satisfactorily delivering hothumid air. The theoretical models coincided well with theexperimental results, verifying the design criteria developedhere. The heat transfer calculations indicated that mostresistance to heat transfer is on the exhaust gas side. Thus, asurface extended tube (Sunrod) was used in the test rig. Itcould be concluded that the tubular humidifier is a strongalternative to the packed-bedtower, especially in smallhigh-pressure gas turbines.Furthermore, the importance of the non-ideality of theair-water vapor mixture in modeling evaporative cycles wasfirst highlighted in this work. Through applying realthermodynamic properties of air-water vapor mixtures in cyclecalculations, it was found that the compressed air contains ahigher amount of moisture than indicated by the ideal gasmixture model. This affects the design of the heat recoverysystem and cannot be neglected.Key words:evaporative gas turbine, indirect-fired gasturbine, humidification, packed bed, tubular humidifier,evaporator, saturator.
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| 25. |
- Danielsson, Erik, 1971-
(författare)
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Processing and characterization of GaN/SiC heterojunctions and SiC bipolar transistors
- 2001
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Silicon Carbide (SiC) and Gallium Nitride (GaN) are bothwide bandgap semiconductors that have been suggested for highpower, high voltage, and high temperature applications. Themost investigated SiC devices so far are the Schottky diode,PiN diode and the field effect transistor. However, untilrecently very few results have been presented for the BipolarJunction Transistor (BJT), which is the focus of this thesis.Since GaN can be grown epitaxially on SiC, the fabrication ofHeterojunction Bipolar Transistors is an interesting path tofurther enhance the operation of the bipolar transistor. Devicesimulations have also been extensively used in the analysis ofthe fabricated structures. Since the processing of wide bandgapdevices is still a relatively immature technology, a bipolarprocess was also developed as part of this thesis.The bipolar transistors were fabricated from n-type wafers,with up to four epitaxial layers grown on the substrate. Theprocess development was mainly focused on dry etching of SiC,passivation of etch damage, and metallization. For the dryetching of SiC an Inductively Coupled Plasma (ICP) etch wasused, and etch rates up to 0.3 µm/min was achieved with SF6as reactive gas. However, the damage increaseswith etch rate, thus the rate was usually lowered to 0.1µm/min for device fabrication. Even at these low etchrates the surface was damaged, which was investigated togetherwith the passivation of the damage. Metallization schemes wereinvestigated for both n-type and p-type SiC.The fabricated BJTs showed a current gain of 10 times and anopen base breakdown voltage up to 600 V. A SiC BJT withimplanted emitter was also fabricated, using phosphorus asimplanted n-type dopant. However, only unity current gain wasobtained on these transistors, and further development of theimplantation process is necessary.The bipolar process was also used to fabricate GaN/SiC HBTs,with the GaN as the top emitter layer. The dry etching of GaNwas performed using a conventional Reactive Ion Etch (RIE). Themetallization to n-type GaN and AlGaN was also developed, andthe Al-Ti alloy worked excellent as ohmic contact, with contactresistivities in the 10-5Ω cm2range. Since these HBTs showed poor transistoroperation, a detailed electrical characterization of GaN/SiCheterojunctions was initiated. The heterojunctions werecharacterized using current-voltage (IV) andcapacitance-voltage (CV) measurements. A model was presentedfor a recombination process assisted by tunneling, which couldexplain the poor operation of the HBTs. The type II (ECfor GaN below ECfor SiC) alignment of the GaN/SiC heterojunctionmay also enhance the recombination process. Differentcompositions of AlGaN were investigated as heterojunctions onSiC, and the Al0.3Ga0.7N, showed a change in the characteristics thatindicated a decreased recombination.Keywords:silicon carbide, gallium nitride, devicesimulation, bipolar junction transistor, heterojunction,heterojunction bipolar transistor, current-voltage measurement,capacitance-voltage measurement, self-heating.
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| 26. |
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| 27. |
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| 28. |
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| 29. |
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| 30. |
- Elger, Ragna
(författare)
-
On the behaviour of the lithium ion battery in the HEV application
- 2004
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The lithium ion battery is today mainly used in cell phonesand laptops. In the future, this kind of battery might beuseful in hybrid electric vehicles as well. In this work, the main focus has been to gain more knowledgeabout the lithium ion battery in the hybrid electric vehicle(HEV) and more precisely to examine what processes of thebattery that are limiting at HEV currents. Both experiments andmathematical modelling have been used. In both cases, highrate, pulsed currents typical for the HEV, have been used. Two manuscripts have been written. Both of them concern thebehaviour of the battery at HEV load, but from different pointsof view. The first one concerns the electrochemical behaviourof the battery at different ambient temperatures. Theexperimental results of this paper were used to validate amathematical model of a Li-ion battery. Possiblesimplifications of the model were identified. In this work itwas also concluded that the mass transfer of the electrolyte isthe main limiting process within the battery. The mass transferof the electrolyte was further studied in the second paper,where the concentration of lithium ions was measured indirectlyusing in situ Raman spectroscopy. This study showed that themathematical description of the mass transfer of theelectrolyte is not complete. One main reason of this issuggested to be the poor description of the physical parametersof the electrolyte. These ought to be further studied in orderto get a better fit between concentration gradients predictedby experiments and model respectively.
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| 31. |
- Enbäck, Sofia
(författare)
-
The anode in a PEFC reformate system
- 2004
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Polymer electrolyte fuel cells (PEFC) constitute one ofseveral types of fuel cells that can be used to converthydrogen to electricity and heat. PEFC co-generation systemsbased on fossil fuels are on the edge of commercialisation.Biogas may be a future fuel in these kinds of systems. Thecommon denominator for these fuels is the carbonmonoxide-containing product gas from the fuel processor,poisoning the catalyst of the fuel cell.A model for the carbon monoxide (CO) oxidation mechanism onPtRu/C in Nafion® was validated by steady-state andopen-circuit potential decay experiments in a porous electrode.Furthermore, a natural gas fuelled system with an autothermalreformer and PEFC has been built from purchased components. Theconstruction was lead by ABB AB with support from KTH,especially in designing the control system. The differentcomponents of the system were evaluated at KTH.It is shown that the oxidation of CO can be described with amodel that takes into consideration the adsorption of CO on Ptand the adsorption of H2O on Ru. The adsorbed CO can be eitherlinearly or bridge-bonded to the surface. The two forms cannotbe distinguished from each other by the method used in thiswork. It is also shown that the oxidation of CO cannot bedescribed by a model describing the surface as homogeneous,where all the catalyst sites are treated equally. Therate-determining step is the oxidation step, with the otherreactions in pseudo-equilibrium up to about 0.5 V vs. RHE,where the adsorption of CO is not in equilibrium but still notrate-determining. However, at the lowest potentials, theadsorption of water might be the rate-determining step. Theopen circuit decay method confirmed the accuracy of thesteady-state measurements. This technique does not suffer fromnoise or the placement of the reference electrode since thepotential gradient in the electrolyte disappears as soon as thecurrent is interrupted. Furthermore, the model presentedpredicts the coverage of COads on the catalyst surface. Thus,the model can be used together with a model for H2 oxidation.The experimental set-up of the single cell is a useful platformfor future work on CO kinetics with different catalysts and atdifferent temperatures. It is possible to study the hydrogenoxidation in presence of CO with existing equipment but purehydrogen oxidation will still be challenging.The PEFC system is based on a fuel processor for 2-10 kWeland a fuel cell stack of 2 kWel maximum. Furthermore, itcontains a DC/AC inverter, a control system, a gas analysissystem and extensive measurement equipment, such asthermocouples and pressure meters. The research PEFC systemwith infrastructure is a useful platform for future researchalthough various components may need to be replaced.
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| 32. |
- Eriksson, Sara, 1977-
(författare)
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Development of catalysts for natural gas-fired gas turbine combustors
- 2006
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Due to continuously stricter regulations regarding emissions from power generation processes, further development of existing gas turbine combustors is essential. A promising alternative to conventional flame combustion in gas turbines is catalytic combustion, which can result in ultralow emission levels of NOx, CO and unburned hydrocarbons. The work presented in this thesis concerns the development of methane oxidation catalysts for gas turbine combustors. The application of catalytic combustion to different combustor concepts is addressed in particular. The first part of the thesis (Paper I) reports on catalyst development for fuel-lean methane combustion. Supported Pd-based catalysts were investigated at atmospheric pressure. The effect on catalytic activity of diluting the reaction mixture with water and/or carbon dioxide was studied in order to simulate a combustion process with exhaust gas recirculation. The catalytic activity was found to decrease significantly in the presence of water and CO2. However, modifying the catalyst by changing support material can have a considerable impact on the performance. In the second part of this thesis (Papers II-IV), the development of rhodium catalysts for fuel-rich methane combustion is addressed. The effect of catalyst composition, oxygen-to-fuel ratio and catalyst pre-treatment on the methane conversion and the product gas composition was studied. An experimental investigation at elevated pressures of partial oxidation of methane/oxygen mixtures in exhaust gas-rich environments was also conducted. The most suitable catalyst identified for fuel-rich catalytic combustion of methane, i.e. Rh/Ce-ZrO2, showed benefits such as low light-off temperature, high activity and enhanced hydrogen selectivity. In the final part of the thesis (Paper V), a numerical investigation of fuel-rich catalytic combustion is presented. Measurements and predictions were compared for partial oxidation of methane in exhaust gas diluted mixtures at elevated pressures. The numerical model was validated for several Rh-based catalysts. The key parameter controlling the catalytic performance was found to be the noble metal dispersion.
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| 33. |
- Eriksson, Sara
(författare)
-
Development of methane oxidation catalysts for different gas turbine combustor concepts
- 2005
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Due to continuously stricter regulations regarding emissions from power generation processes, development of existing gas turbine combustors is essential. A promising alternative to conventional flame combustion in gas turbines is catalytic combustion, which can result in ultra low emission levels of NOx, CO and unburned hydrocarbons. The work presented in this thesis concerns the development of methane oxidation catalysts for gas turbine combustors. The application of catalytic combustion to different combustor concepts is addressed in particular. The first part of the thesis (Paper I) reports on catalyst development for fuel-lean methane combustion. The effect on catalytic activity of diluting the reaction mixture with water and carbon dioxide was studied in order to simulate a combustion process with exhaust gas recirculation. Palladium-based catalysts were found to exhibit the highest activity for methane oxidation under fuel-lean conditions. However, the catalytic activity was significantly decreased by adding water and CO2, resulting in unacceptably high ignition temperatures of the fuel. In the second part of this thesis (Paper II), the development of rhodium catalysts for fuel-rich methane combustion is addressed. The effect of water addition on the methane conversion and the product gas composition was studied. A significant influence of the support material and Rh loading on the catalytic behavior was found. The addition of water influenced both the low-temperature activity and the product gas composition.
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| 34. |
- Ersson, Anders, 1971-
(författare)
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Materials for High-Temperature Catalytic Combustion
- 2003
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Catalytic combustion is an environmentally friendlytechnique to combust fuels in e.g. gas turbines. Introducing acatalyst into the combustion chamber of a gas turbine allowscombustion outside the normal flammability limits. Hence, theadiabatic flame temperature may be lowered below the thresholdtemperature for thermal NOXformation while maintaining a stable combustion.However, several challenges are connected to the application ofcatalytic combustion in gas turbines. The first part of thisthesis reviews the use of catalytic combustion in gas turbines.The influence of the fuel has been studied and compared overdifferent catalyst materials.The material section is divided into two parts. The firstconcerns bimetallic palladium catalysts. These catalysts showeda more stable activity compared to their pure palladiumcounterparts for methane combustion. This was verified both byusing an annular reactor at ambient pressure and a pilot-scalereactor at elevated pressures and flows closely resembling theones found in a gas turbine combustor.The second part concerns high-temperature materials, whichmay be used either as active or washcoat materials. A novelgroup of materials for catalysis, i.e. garnets, has beensynthesised and tested in combustion of methane, a low-heatingvalue gas and diesel fuel. The garnets showed some interestingabilities especially for combustion of low-heating value, LHV,gas. Two other materials were also studied, i.e. spinels andhexaaluminates, both showed very promising thermal stabilityand the substituted hexaaluminates also showed a good catalyticactivity.Finally, deactivation of the catalyst materials was studied.In this part the sulphur poisoning of palladium, platinum andthe above-mentioned complex metal oxides has been studied forcombustion of a LHV gas. Platinum and surprisingly the garnetwere least deactivated. Palladium was severely affected formethane combustion while the other washcoat materials were mostaffected for carbon monoxide and hydrogen.Keywords:catalytic combustion, catalyst materials,palladium, platinum, bimetallic, garnet, spinel, hexaaluminate,deactivation, sulphur, poisoning, diesel, methane,hydrocarbons
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| 35. |
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| 36. |
- Ferrandon, Magali
(författare)
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Mixed metal oxide - noble metal catalysts for total oxidation of volatile organic compounds and carbon monoxide
- 2001
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- CO, volatile organic compounds, and polyaromatics areubiquitous air pollutants that give rise to deleterious healthand environmental effects. Such compounds are emitted, forinstance, by the combustion of wood, particularly fromsmall-scale heating appliances. Total catalytic oxidation isconsidered to be an effective approach in controlling theseemissions, however, some problems remain such as thenon-availability of catalysts with low-cost, high activity andstability in prevailing conditions. Hence, this thesis aims atthe development of oxidation catalysts and improvedunderstanding of their behaviour.The catalytic activity was evaluated for the oxidation of amixture of CO, naphthalene (or ethylene), and methane inpresence of carbon dioxide, water, oxygen and nitrogen. Variouscharacterisation techniques, including Temperature-ProgrammedReduction and Oxidation, BET-Surface Area Analysis, X-RayDiffraction, X-Ray Photoelectron Spectroscopy, RamanSpectroscopy and Scanning and Transmission Electron Microscopywere used.In the first part of this thesis, catalysts based on metaloxides (MnOx, CuO) and/or a low amount of noble metals (Pt, Pd)supported on alumina washcoat were selected. It was shown thatPt and Pd possessed a superior catalytic activity to that ofCuO and MnOxfor the oxidation of CO, C10H8and C2H4, while for the oxidation of CH4, CuO was largely more active than noble metals,and MnOxas active as Pd and Pt. Some mixed metaloxide-noble metal catalysts showed decreased activity comparedto that of noble metals, however, a higher noble metal loadingor a successive impregnation with noble metals led to positivesynergetic effects for oxidation.Deactivation of the catalysts by thermal damage and sulphurpoisoning is addressed in the second part of the dissertation.An alumina washcoat was found to be well anchored to themetallic support after thermal treatment at 900°C due tothe growth of alumina whiskers. The sintering of the washcoatwas accelerated after high temperature treatments in thepresence of metal catalysts. In addition, alumina was found toreact with CuO, particularly in presence of noble metals at900°C, to form inactive CuAl2O4. However, MnOxcatalyst benefits from the more active Mn3O4phase at high temperature, which makes it asuitable active catalyst for the difficult oxidation of CH4. Pt sintering was delayed when mixed with CuO,thus giving more thermally resistant catalyst. The mixed metaloxide-noble metal catalysts showed higher activity afterpre-sulphation of the catalysts with 1000 ppm SO2in air at 600°C or during activitymeasurement in presence of 20 ppm SO2in the gas mixture, compared to single componentcatalysts. In some cases, the activities of the mixed catalystswere promoted by pre-sulphation due to the presence of sulphatespecies.Thermal stabilisation of the catalytic componentsand thealumina by promotion of La in the washcoat is discussed in thethird section. The stabilising effect of La at high temperatureis also compared to that of Ce added in the catalysts for otherpurposes. Due to its better dispersion, La contributed to thethermal stabilisation of the alumina washcoat and its activecomponents to a higher extent than Ce did. La provided a betterdispersion and a higher saturation of metal oxides in thealumina support, and at the same time stabilised the activityof the catalysts by preventing undesirable solid-phasereactions between metal oxide and alumina. In addition, La wasfound to enhance the dispersion and the oxygen mobility of CeO2. Cu-Ce interactions were found to promotesubstantially the CO oxidation due to an increase of thestability and reducibility of Cu species. Synergetic effectswere also found between Ce and La in the washcoat of CuO-Ptcatalyst, which facilitated the formation of reduced Pt and CeO2, thus enhancing significantly the catalyticactivity compared to that of a Pt only catalyst.The last part was an attempt to demonstrate the potential ofa catalyst equipped with a pre-heating device in a full-scalewood-fired boiler for minimising the high emissions during thestart-up phase. During the first ten minutes of the burningcycle a significant reduction of CO and hydrocarbons wereachieved.Keywords: wood combustion, catalysts, total oxidation,manganese, copper, platinum, palladium, lanthanum, cerium, CO,VOC, methane, deactivation, thermal stability, sulphurdioxide.
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| 37. |
- Fillman, Benny, 1969-
(författare)
-
System studies of MCFC power plants
- 2005
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Die Brennstoffzelle ist ein elektrochemischer Reaktor und wandelt chemisch gebundene Energie direkt in elektrische Energie um. In der stationären Energieerzeugung ist der Brennstoffzellenstapel selbst nur ein kleiner Bestandteil des vollständigen Systems. Die Integration aller zusätzlichen Bestandteile, der Peripheriegeräte (Balance-of-Plant) (BoP), ist eine der Hauptaufgaben in der Studie der Brennstoffzellenkraftwerke. Diese Untersuchung betrifft die Systemstudie des auf der Schmelz-Karbonat-Brennstoffzelle (MCFC) basierten Kraftwerks. Die Systemstudie ist mit dem Simulationprogramm Aspen PlusTM durchgeführt worden. Artikel I beschreibt die Implementierung eines in Aspen PlusTM entwickelten MCFC Stapelmodells, um ein MCFC Kraftwerk zu studieren, das Erdgas als Brennstoff verwendet. Artikel II beschreibt, wie unterschiedliche Prozeßparameter, wie Brenngasnutzung und dieWahl des Brennstoffes, die Leistung eines MCFC Kraftwerks
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| 38. |
- Gamero, Rafael
(författare)
-
Mass transfer during isothermal drying of a porous solid containing multicomponent liquid mixtures
- 2004
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Mass transfer in a porous solid, partially saturated with asingle solvent and multicomponent liquid mixtures, has beenexperimentally and theoretically studied. A porous materialcontaining single liquids and mixtures of organic solvents wasisothermally dried. Experiments were performed using a jacketedwind tunnel, through which a humidity andtemperature-controlled air stream flowed. The wetted porousmaterial was placed in a cylindrical vessel, whose top isexposed to the air stream until the material became dried to acertain extent. Drying experiments with the single solventswater, methanol, ethanol and 2-propanol, were performed atdifferent temperatures and transient liquid content profileswere determined. In isothermal drying experiments with liquidmixtures,the transient concentration profiles of thecomponents along the cylindrical sample as well as the totalliquid content were determined. The liquid mixtures examinedwere water-methanolethanol and isopropanol-methanol-ethanol.Two different temperatures and initial compositions were usedin the experiments. Mathematical models that describe nonsteadystate isothermal drying of a solid containing single liquidsand multicomponent liquid mixtures were developed. In the solidwetted with a single liquid, capillary movement of the liquidwas the main mechanism responsible for mass transfer. In thesolid containing liquid mixtures, interactive diffusion inliquid phase was superimposed to the capillary movement of theliquid mixture. In addition, interactive diffusion of thevapours in empty pores was considered. The parameters todescribe the retention properties of the solid and thecapillary movement of the liquid were determined by comparingtheoretical and experimental liquid content profiles obtainedduring drying of the solid wetted with single liquids. Tosimulate the transport of the liquid mixtures these parameterswhere weighed according to liquid composition. A fairly goodagreement between theoretical and experimental liquidcomposition profiles was obtained if axial dispersion isincluded in the model when the moisture consists of amixture. Keywords:Internal mass transfer, capillary flow,multicomponent, diffusion, solvent mixtures
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| 39. |
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| 40. |
- Ganemi, Bager
(författare)
-
Zeolite ZSM5 catalysts for abatement of nitrogen oxide
- 1999
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Airborne pollutants from the combustion of fossil fuels area global problem. Emission of nitrogen oxides (NOx) is increasing with the worldwide increase in theuse of energy. Atmospheric and photochemical reactions linknitrogen oxides to hydrocarbons and tropospheric ozone. Theemission of NOxhas to be tackled urgently in order to limit theharmful effects of anthropogenic activity on theenvironment.The subject of this thesis is catalytic nitrogen oxideabatement through direct decomposition and reduction by methaneover ion-exchanged zeolite ZSM5. The work covers catalyticconversionand surface intermediates, including correlationswith the level of exchanged Cu2+cations and Ni2+or Pd2+co-cations. Special attention is given to thealuminium content of the support and changes in structuralparameters. It was found that NOxconversion over cation-exchanged ZSM5 is stronglyinfluenced by the ion-exchange procedure and by the abovematerial parameters.Characterization of Cu-ZSM5 reveals that approximately twomolecules of water per Cu2+ion desorb at temperatures between 150 and 350oC, in addition to the conventional dehydration atlower temperatures. The desorbed water comes from thedecomposition of Cu(OH)2. Decomposition of hydroxylated copper ions resultsin the formation of Cu2+-O-Cu2+dimers, which are suggested to be the activesites for catalytic decomposition of NO.Acid sites are important for the dispersion of copper ionson the catalyst surface. Acid sites are also important for theinteraction between copper species and the zeolite. Increasedacidity leads to a stronger interaction between the exchangedcation and the framework, i.e. the exchanged cations becomemore resistant to mobility. The stronger bond between theexchanged cations and lattice oxygen also preventsdealumination of the catalyst and decreases the thermalexpansion at higher temperatures.The temperature of dehydroxylation of acid sites on H-ZSM5overlaps with the light-off temperature for NO reduction overCu-ZSM5. Bridged nitrato groups ligated to Cu2+-O-Cu2+dimers act as site blockers below the light-offtemperature. At the light-off temperature zeolite latticevibrations destabilize surface nitrates and open the sites forcatalytic reactions via short-lived N2O3intermediates.The same lattice movementsdecompose OH-groups on the H-form of the zeolite and it wassuggested that zeolite ZSM5 should be noted mainly for itsflexibility rather than its narrow channels with strongelectrostatic fields or metal exchange sites with opencoordination.Keywords:NO, NO2, Decomposition, Reduction, Methane, ZSM5,Ion-exchange, Cu, Pd, Ni
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| 41. |
- García, Indiana, 1966-
(författare)
-
Removal of natural organic matter by enhanced coagulation in Nicaragua
- 2005
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The existence of trihalomethanes (THMs) in a drinking water plant of Nicaragua has been investigated in order to see whether the concentration exceeded the maximum contaminant level recommended by the environmental protection agency of the United States (USEPA) and the Nicaragua guidelines. The influence of pH, temperature, chlorine dose and contact time on the formation of THMs were studied. The contents of organic matter measured by surrogate parameters such as total organic carbon, dissolved organic carbon, ultraviolet absorbance and specific ultraviolet absorbance were also determined in order to show which type of organic matter is most reactive with chlorine to form THMs. Models developed by other researchers to predict the formation of trihalomethanes were tested to see whether they can be used to estimate the trihalomethane concentration. In addition, empirical models were development to predict the THM concentration of the drinking water plant analysed. The raw water was treated by conventional and enhanced coagulation and these processes were compared with regard to the removal of natural organic matter (NOM). The significance of the results was assessed using statistic procedures. The average concentration of THMs found at the facility is below the USEPA and Nicaragua guideline values. Nevertheless the maximum contaminant level set by USEPA is sometimes exceeded in the rainy season when the raw water is rich in humic substances. Comparison between the water treated by conventional and enhanced coagulation shows that enhanced coagulation considerably diminished the trihalomethane formation and the value after enhanced coagulation never exceeded the guidelines. This is because enhanced coagulation considerably decreases the organic matter due to the high coagulant dose applied. The study of the trihalomethane formation when varying pH, time, temperature and chlorine dose using water treated by conventional and enhanced coagulation showed that higher doses of chlorine, higher pH, higher temperature and a longer time increases the formation of THMs. However, combinations of two and three factors are the opposite. The predicted THM formation equations cannot be used for the water at this facility, since the results shown that the measured THM differs significantly from the THM concentration predicted. Two empirical models were developed from the data for enhanced coagulation, using linear and non-linear regression. These models were tested using the database obtained with conventional coagulation. The non-linear model was shown to be able to predict the formation of THMs in the Boaco drinking water plant.
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| 42. |
- Georén, Peter, 1972-
(författare)
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Characterisation and modelling of lithium-ion battery electrolytes
- 2003
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Rechargeable batteries play an important role as energycarriers in our modern society, being present in wirelessdevices for everyday use such as cellular phones, video camerasand laptops, and also in hybrid electric cars. The batterytechnology dominating the market today is the lithium-ion(Li-ion) battery. Battery developments, in terms of improvedcapacity, performance and safety, are major tasks for bothindustry and academic research. The performance and safety ofthese batteries are greatly influenced by transport andstability properties of the electrolyte; however, both haveproven difficult to characterise properly.The specific aim of this work was to characterise and modelthe electrolytes used in Li-ion batteries. In particular, themass transport in these electrolytes was studied throughcharacterisation and modelling of electrolyte transport in bulkand in porous electrodes. The characterisation methodology assuch was evaluated and different models were tested to find themost suitable. In addition, other properties such aselectrochemical stability and thermal properties were alsostudied.In the study of electrochemical stability it wasdemonstrated that the electrode material influenced thevoltammetric results significantly. The most versatileelectrode for probing the electrolyte stability proved to beplatinum. The method was concluded to be suitable for comparingelectrolytes and the influences of electrolyte components,additives and impurities, which was also demonstrated for a setof liquid and polymer containing electrolytes.A full set of transport properties for two binary polymerelectrolytes, one binary liquid and the corresponding ternarygel were achieved. The transport was studied both in the bulkand in porous electrodes, using different electrochemicaltechniques as well as Raman spectroscopy. In general, theconductivity, the salt and solvent diffusivity decreasedsignificantly when going from liquid to gel, and to polymerelectrolyte. Additionally, low cationic transport numbers wereachieved for the polymer and gel and significant salt activityfactor variations were found. The results were interpreted interms of molecular interactions. It was concluded that both theionic interactions and the influences from segmental mobilitywere significant for the polymer containing electrolytes. Thecharacterisation methods and the understanding were improved bythe use of a numerical modelling using a model based on theconcentrated electrolyte theory. It was concluded thatelectrochemical impedance spectroscopy and Raman spectroscopywere insufficient for determining a full set of transportproperties. It was demonstrated that the transport is veryinfluential on electrochemical impedance as well as batteryperformance.Keywords:lithium battery, electrolyte, mass transport,stability, modelling, characterisation, electrochemical, Ramanspectroscopy, impedance
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| 43. |
- Gode, Peter, 1973-
(författare)
-
Investigations of proton conducting polymers and gas diffusion electrodes in the polymer electrolyte fuel cell
- 2005
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Polymer electrolyte fuel cells (PEFC) convert the chemically bound energy in a fuel, e.g. hydrogen, directly into electricity by an electrochemical process. Examples of future applications are energy conversion such as combined heat and power generation (CHP), zero emission vehicles (ZEV) and consumer electronics. One of the key components in the PEFC is the membrane / electrode assembly (MEA). Both the membrane and the electrodes consist of proton conducting polymers (ionomers). In the membrane, properties such as gas permeability, high proton conductivity and sufficient mechanical and chemical stability are of crucial importance. In the electrodes, the morphology and electrochemical characteristics are strongly affected by the ionomer content. The primary purpose of the present thesis was to develop experimental techniques and to use them to characterise proton conducting polymers and membranes for PEFC applications electrochemically at, or close to, fuel cell operating conditions. The work presented ranges from polymer synthesis to electrochemical characterisation of the MEA performance. The use of a sulfonated dendritic polymer as the acidic component in proton conducting membranes was demonstrated. Proton conducting membranes were prepared by chemical cross-linking or in conjunction with a basic functionalised polymer, PSU-pyridine, to produce acid-base blend membranes. In order to study gas permeability a new in-situ method based on cylindrical microelectrodes was developed. An advantage of this method is that the measurements can be carried out at close to real fuel cell operating conditions, at elevated temperature and a wide range of relative humidities. The durability testing of membranes for use in a polymer electrolyte fuel cell (PEFC) has been studied in situ by a combination of galvanostatic steady-state and electrochemical impedance measurements (EIS). Long-term experiments have been compared to fast ex situ testing in 3 % H2O2 solution. For the direct assessment of membrane degradation, micro-Raman spectroscopy and determination of ion exchange capacity (IEC) have been used. PVDF-based membranes, radiation grafted with styrene and sulfonated, were used as model membranes. The influence of ionomer content on the structure and electrochemical characteristics of Nafion-based PEFC cathodes was also demonstrated. The electrodes were thoroughly investigated using various materials and electrochemical characterisation techniques. Electrodes having medium Nafion contents (3545 wt %), the cathode becomes limited by diffusion of O2 both in the agglomerates and throughout the cathode. Furthermore, models for the membrane coupled with kinetics for the hydrogen electrode, including water concentration dependence, were developed. The models were experimentally validated using a new reference electrode approach. The membrane, as well as the hydrogen anode and cathode characteristics, was studied experimentally using steady-state measurements, current interrupt and EIS. Data obtained with the experiments were in good agreement with the modelled results.
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| 44. |
- Gracin, Sandra
(författare)
-
Polymorphism and Crystallization of p-Aminobenzoic Acid
- 2004
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Polymorphs are solid phases where the chemical composition isequal but the crystal structure differs. Many organic compoundsmay appear in more than one crystalline structure. The differentpolymorphs of a givensubstance may have significantly differentphysical properties (packing, thermodynamic, spectroscopic,kinetic, surface and mechanical properties). Sometimes the moststable polymorph is difficult to produce or a metastable form hasfavourable properties. This thesis focuses on the crystallization of p-aminobenzoicacid. This model compound crystallizes in two differentpolymorphic forms: 1) the α-polymorph, which isthe commercially available form and appears as long, fibrousneedles 2) theβ-polymorph, that appears in the form ofprisms. The thermodynamic stability and crystallization from differentsolvents have been studied experimentally. The system is found tobe enantiotropic with a transition temperature of 25 °C,below which theβ-form is the stable polymorph. The compoundhas been crystallized from thirteen different solvents, either byslow cooling after which the product is allowed to mature insuspension, or by rapid cooling followed by immediate isolation.Needles were obtained from all solvents by both methods. In waterand ethyl acetate below 20°C the prismaticβ-form isobtained however, often together with the needles. Theβ-formcrystals usually needed hours or days to grow at the very slowcooling used, while needles usually appeared in seconds. Bycareful control of supersaturation and temperature coolingcrystallization can be performed to produce the pure β-formin water and in ethyl acetate. The influence of the solvent isexplained by analysis of the crystal structures versus thepossible interaction of the solvent molecules with the solute insolution. The α-form structure is governed by dimers and iskinetically favoured because the dimers easily form in thesolution especially in less polar solvents. The crystal structureof the β-modification is not based on dimers but on fourmembered rings with alternating amino and carboxyl groups. Keywords:controlled crystallization, polymorphism,p-aminobenzoic acid
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| 45. |
- Gracin, Sandra, 1968-
(författare)
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Solubility and polymorphism of molecular compounds
- 2005
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis deals with the controlled crystallization of small organic molecules and is focused on solubility and polymorphism. The solubility was determined for phenylacetic acid, p-hydroxyphenylacetic acid, p-aminophenylacetic acid, p-hydroxybenzoic acid and ibuprofen in both water and in a range of organic solvents. Data is discussed from the standpoint of molecular aspects of solute – solvent interactions and by estimated solid phase activity. It was shown that better understanding could be acquired by making a qualitative analysis of the molecular interactions in the solution and the crystal structure of the compounds in question. Solubility predictions that are carried out by the UNIFAC method are not sufficiently accurate to serve as a basis for a reliable design of a crystallization process or selection of a suitable solvent since they deviate more than 15% from experimental values. The reason for the discrepancies are related to uncertainties in the prediction of activity coefficients by UNIFAC, as well as, difficulties in the estimation of the activity of the solid state. p-Aminobenzoic acid (PABA) has been crystallized from thirteen different solvents either by slow cooling, after which the product is allowed to mature in suspension, or by rapid cooling followed by immediate isolation. Two different polymorphs have been crystallized. The system is found to be enantiotropic with the transition temperature of 25 °C, below which the β-form is the stable polymorph. The α-form was obtained from all solvents by both methods. The β-form is obtained only in carefully controlled conditions from water and ethyl acetate, well below the transition temperature. Often the α-form appears concomitantly. It is shown in this work that sonication significantly reduces the induction time for nucleation. The β-form crystallizes more reproducibly and at higher cooling rates when controlled sonication is used. In addition sonication is found to selectively favor the appearance of one of the polymorphs. Producing the pure β-form was possible even above the transition temperature where other crystallization techniques were only capable of producing the stable α-form. The α-form structure is based on centro symmetric dimers formed by association of carboxylic acid groups. It is suggested that the preference for nucleation of the α-polymorph is related to the formation of dimers in the supersaturated solution. Only at the condition where the formation of dimers is reduced sufficiently, (i.e. in the polar solvents or when sonication is applied) the nucleation of the β-form is favored.
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- Grönkvist, Stefan
(författare)
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All CO2 molecules are equal, but some CO2 molecules are more equal than others
- 2005
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis deals with some challenges related to the mitigation of climate change and the overall aim is to present and assess different possibilities for the mitigation of climate change by: • Suggesting some measures with a potential to abate net greenhouse gas (GHG) emissions, • Discussing ideas for how decision-makers could tackle some of the encountered obstacles linked to these measures, and • Pointing at some problems with the current Kyoto framework and suggesting modifications of it. The quantification of the net CO2 effect from a specific project, frequently referred to as emissions accounting, is an important tool to evaluate projects and strategies for mitigating climate change. This thesis discusses different emissions accounting methods. It is concluded that no single method ought to be used for generalisation purposes, as many factors may affect the real outcome for different projects. The estimated outcome is extremely dependent on the method chosen and, thus, the suggested approach is to apply a broader perspective than the use of a particular method for strategic decisions. The risk of losing the integrity of the Kyoto Protocol when over-simplified emissions accounting methods are applied for the quantification of emission credits that can be obtained by a country with binding emissions targets for projects executed in a country without binding emission targets is also discussed. Driving forces and obstacles with regard to energy-related co-operations between industries and district heating companies have been studied since they may potentially reduce net GHG emissions. The main conclusion is that favourable techno-economic circumstances are not sufficient for the implementation of a co-operation; other factors like people with the true ambition to co-operate are also necessary. How oxy-fuel combustion for CO2 capture and storage (CCS) purposes may be much more efficiently utilised together with some industrial processes than with power production processes is also discussed. As cost efficiency is relevant for the Kyoto framework, this thesis suggests that CCS performed on CO2 from biomass should be allowed to play on a level playing field with CCS from fossil sources, as the outcome for the atmosphere is independent of the origin of the CO2.
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